When distilling and refining easily polymerizable substances, for example, acrylic acid and methacrylic acid or their esters, distillation is performed with addition of polymerization inhibitors and under reduced pressure in order to prevent polymerization from occurring. However, it is impossible to completely prevent polymerization and it is inevitable that some polymerization occurs. In particular, for example, a drastic change in conditions under which a distillation apparatus is operated for any reason may result in an abrupt progress in polymerization. Therefore, some polymerisate may exist in distillation residue drawn out from a bottom of the distillation apparatus and polymerisate may also be contained in condensate in a reflux tank at a top of the apparatus.
Extracting distillation residues at a bottom of the column to an exterior, extracting condensate in the reflux tank at a top of the column to the exterior, or refluxing the condensate to a distillation column needs aid of a pump. However, direct supply of a liquid that contains solids such as a polymerisate may cause solids to be attached to a casing part of the pump or enter into a mechanical seal part of the pump. Once such a phenomenon occurs, flow is stagnated at that portion, so that polymerization of a polymerizable substance in the distillation residue proceeds, thus causing a serious problem in operation of the pump, and ultimately it becomes inevitable to stop the operation of the pump.
Further, it is possible that solids that passed the pump may cause clogging of a flow meter and control valve provided on a discharge side of the pump. Therefore, a strainer is provided usually on an upstream side of the pump in order to avoid feeding of solids to the pump. To allow for continued operation of the distillation column even when the strainer is clogged with solids, two or more strainers are provided in parallel. In a case where one strainer which is in operation is clogged, a backup strainer provided in parallel can be immediately brought into operation by switching valves.
However, in general, switching a strainer of a pump for extracting a liquid from a vacuum distillation apparatus to a backup strainer tends to cause an abnormal operation of the pump. Also, in a case of distillation and refining of a polymerizable substance, switching strainers of a pump for extracting a condensate from a reflux tank may result in production of a polymerisate in the condensate in the reflux tank. Also in a case of switching the strainers of a pump for extracting distillation residue from a bottom of a column, production of polymerisate in the column may increase.
The inventors of the present invention have studied a cause therefor and as a result, they have found that such is attributable to air contained in the backup strainer. That is, a strainer clogged with solids becomes a backup strainer to be opened and cleaned. These operations are performed in air and hence atmospheric air is present in the backup strainer. Accordingly, operation of the backup strainer will result in back flow of inside air to an upstream side to flow into the apparatus under reduced pressure, or flow into the pump located downstream, or further into an apparatus to which a discharge liquid of the pump flows. As a result, operation of the pump is disturbed and the operation of the pump must be stopped, as the case may be.
Further, this disturbed operation of the pump or flowed in air will disturb flow of steam and liquid within the distillation apparatus, with a result that polymerization of a polymerizable substance is promoted. That is, although a polymerization inhibitor is fed to a condenser at the top of the column to prevent the condensate from polymerizing when distilling and refining a polymerizable substance, it is conceivable that disturbed flow of steam in the condenser will produce a condensate in which concentration of the polymerization inhibitor is locally reduced, which triggers polymerization.
Further, back flow into the distillation column of the air in the backup strainer of a pump for extracting distillation residues from the bottom of a column will disturb the flow of the steam and liquid in the column, causing local stagnation of the liquid or local concentration reduction of the polymerization inhibitor to promote polymerization.
In a process where acrylic acid, methacrylic acid, and esters thereof, which are typically easily polymerizable substances, are produced by catalytic gas phase oxidation, it is important to stabilize operational conditions of respective unit operation apparatuses used in unit operations such as separation or purification of acrylic acid, methacrylic acid and esters thereof, from a viewpoint of stable production of these substances, which are easily polymerizable substances, in order to suppress formation of polymerisate that will frequently occur during the operation.
Since these substances are easily polymerizable substances, polymerisate is generated inside of a plant in spite of use of a polymerization inhibitor or a polymerization retardant. For example, polymerisate is produced inside each distillation column and condenser that constitute a process, and the polymerisate is accumulated inside of the distillation column, or a part of or all of the polymerisate falls to the bottom of the column, or clogs the condenser, or falls to the reflux tank or the pump connected to the reflux tank through a line.
To remove the polymerisate, for example, a strainer installed on a feed side of a pump for transferring a column bottom liquid that transfers the column bottom liquid to a downstream appliance is provided for purposes of protecting the pump, preventing polymerization solids from being fed to the appliance downstream of the pump, and the like. Further, since continuous operation is intended, two or more strainers parallelly installed (one strainer being in action and other strainer(s) being in a standby state) are provided.
In a case where an easily polymerizable substance is produced, although polymerisate generated can be collected by the strainer and removed from the process thereby, times of switching to the strainer provided as a backup cannot be judged since a state of the polymerisate being collected during the operation of the strainer cannot be grasped. That is, an increased amount of collection will cause clogging of the strainer with collected polymerizate, thus generating cavitation in the pump connected to the strainer. This causes abnormality of pump operation, which in turn leads to disturbed operational conditions (for example, variation in pressure, variation in flow rate, and variation in temperature) in appliances installed on upstream and downstream sides of the pump, thereby generating polymerisate in appliances to stop operation of the appliances or the plant in its entirety.
For this reason, the strainers are frequently switched to cope with the aforementioned problems; however, this incurs a labor load to an operator who operates, and in a case where a substance to be handled is a dangerous substance, toxic substance, or the like, utmost care must be taken and the operator is put under a great psychological burden.
Easily polymerizable substances, particularly acrylic acid, methacrylic acid, and esters thereof have this tendency to such a significant extent. And in an industrial production plant, a countermeasure is demanded to reduce a burden on the operator and avoid a situation where the operation has to be stopped.